DISTRIBUCIÓN DE ALGUNOS ELEMENTOS MAYORES (K, Ca, Ti, Fe) Y TRAZA (Ga,
Rb, Sr, Y, Zr, Br) EN DOS SUELOS POLICÍCLICOS
PODSÓLICOS.
E. PEITEADO VARELA, R. PIÑEIRO REBOLO, A. MARTÍNEZ CORTIZAS
Departamento de Edafología y Química Agrícola, Universidad de Santiago de Compostela, Campus Sur s/n, 15782 Santiago de Compostela
Abstract
Two polycyclic podzolic soils from northwestern Spain were analyzed
for some major and trace elements. Pena Cebral (PC) is located in Serra dos
Ancares and is composed by a basal mixed coluvium of shales and phylites, buried
by a quartzitic coluvium; while Curro Vello II (CVII) is situated in the Serra
do Xistral and developed on a granodioritic coluvium. The vertical distribution
of four major (K, Ca, Ti, Fe) and 6 trace elements (Ga, Rb, Sr, Y, Zr y Br)
in the fine earth (FE, <2mm)
and the silt+clay fraction (SC, <50 µm) was studied using detailed
concentrations profiles, concentration ratios between fractions and enrichment
factors. The concentrations obtained for the fine earth of PC were: K 1.9-4.0%;
Ca is below the detection limit (<0.01%) in all samples except for the upper
most one (O horizon); Ti 0.3-0.5%; Fe 1.4-4.1%; Ga 9.7-18.3 mg kg-1; Rb 87.3-139.6
mg kg-1; Sr 20.8-48 mg kg-1; Y 20-50.8 mg kg-1; Zr 271.4-630 mg kg-1; Br 2.8-30
mg kg-1. In CVII the concentrations determined ranged: K 1.3-3.3%; Ca 0.14-0.25%;
Ti 0.08-0.25%; Fe 1.0-3.1%; Ga 11.1-21.2 mg kg-1; Rb 84.9-132.4 mg kg-1; Sr
41-52 mg kg-1; Y 9.6-53.9 mg kg-1; Zr 75.7-205 mg kg-1; Br 42.1-362.9 mg kg-1.
Both soils have lower average concentrations of Ca and Sr and higher Fe in
CVII and Zr in PC than those reported by other researchers for the parent materials
of the areas where they are located. All the elements, exception made of the
K, Ca and Sr in CVII, showed higher concentrations in the SC than in the FE
and than those estimated for the sands (up to 4.5 times in PC a67nd 2.5 times
in CVII).
Both the distribution profiles and the fractionation of elements seems to be
the result of the interplay of several factors. The influence of the parent
material mineralogy is expresed in higher Sr concentrations in CVII than in
PC while the opposite is true for K, Ti, Fe, Zr, Y and the proportion of fine
material (igneous
material versus metamorphic material). The higher concentrations of the elements
in the SC, particularly for the more mobile ones (K, Ca, Sr), in PC soil indicate
a lower degree of evolution. In CVII these elements are depleted in the SC
indicating a greater degree of chemical evolution, showing in both cases the
effect of the geochemical environment (weathering and pedogenesis). Results
also indicate that Fe and Br, and to some extent Ti, are mobile in these soils
due to podzolitation. The accumulation of Br in the spodic horizons suggests
that the element is incorporated into the soil organic matter in the surface
horizons and transported down in the profile by mobile organic compounds, something
which is consistent with recent work on the halogenation of the soil organic
matter. Thus, Br seems to be a good indicator of the intensity of the podzolisation,
although further research is necessary to elucidate the mechanisms. On the
other hand,this element is subjected to a pronounced source effect, with concentrations
15 times higher in CVII than in PC, as it should be expected given the proximity
to the ocean of the former soil.
Key words: polycyclic soils, podzolisation, major elements, trace elements, bromine.